Nitrogen rejection from natural gas

Abstract

A process is disclosed for rejecting nitrogen from a natural gas feed containing nitrogen over a wide range of compositions, e.g. 5-85% nitrogen by volume, under elevated pressure using a single distillation column and a closed loop methane heat pump which reboils and refluxes the column. An intermediate reflux condenser is refrigerated by both the heat pump and overhead nitrogen fraction from the distillation column. The process can handle feeds with increasing nitrogen content and more than 100 ppmv carbon dioxide. The feed can be at pipeline pressure with the natural gas liquid components still present or at lower pressure with natural gas liquids removed. A mixed cryogenic refrigerant can be used in the heat pump as an alternative to methane. The process provides a high methane recovery over the entire feed range, and provides a nitrogen product stream having an elevated pressure suitable for recycling and reinjection to an oil or gas well to improve well head pressure.

What is claimed is: 
     
       1. A process for separating nitrogen from a high pressure feed containing natural gas and nitrogen over a wide concentration range of 5-85% by volume in a single distillation column to form high pressure product streams of nitrogen and of natural gas comprising: (a) cooling said high pressure feed against high pressure product streams to separate multiple feeds to said column and distilling said cooled feeds in said single distillation column to form a high pressure overhead vapor rich in nitrogen and a high pressure bottoms liquid rich in natural gas hydrocarbons;   (b) condensing a head vapor of an upper section of said column by heat exchange with a first closed loop refrigerant to provide reflux to said column;   (c) condensing an intermediate vapor of an intermediate section of said column by heat exchange with a second closed loop refrigerant and by heat exchange with said high pressure overhead vapor rich in nitrogen without expansion to provide an intermediate reflux to said column, wherein said intermediate section vapor condensing attributable to heat exchange with said high pressure nitrogen overhead increases with increasing nitrogen concentration in said feed.   
     
     
       2. A process according to claim 1 wherein said product stream nitrogen is reinjected into an oil or gas well to improve well head pressure. 
     
     
       3. In a process for separating nitrogen from a high pressure feed containing natural gas and nitrogen consisting of cooling said high pressure feed and distilling the cooled feed in a single distillation column to form a high pressure overhead vapor rich in nitrogen and a high pressure bottoms liquid rich in natural gas hydrocarbons, the improvement for accommodating a wide concentration range of nitrogen of 5-85% by volume in said feed and forming a high pressure product stream of nitrogen suitable for reinjection to maintain a well head pressure for enhanced oil or gas recovery comprising: (a) cooling said high pressure feed against high pressure product streams to separate multiple feeds to said column;   (b) condensing a head vapor of an upper section of said column by heat exchange with a first closed loop refrigerant to provide reflux to said column; and   (c) condensing an intermediate vapor of an intermediate section of said column by heat exchange with a second closed loop refrigerant and by heat exchange with said high pressure overhead vapor rich in nitrogen without expansion to provide an intermediate reflux to said column, wherein said intermediate section vapor condensing attributable to heat exchange with said high pressure nitrogen overhead increases with increasing nitrogen concentration in said feed.   
     
     
       4. A process according to claims 1 or 3 wherein said first and second closed loop refrigerants comprise first and second portions of a circulating refrigeration fluid in a closed loop heat pump wherein said refrigeration fluid is compressed; cooled; condensed in heat exchange with the bottoms in said column, thereby providing reboiler heat to said column; subcooled to a temperature sufficient to form said second closed loop refrigerant; and further subcooled to a temperature sufficient to form said first closed refrigerant. 
     
     
       5. The process according to claim 4 wherein said feed cooling comprises cooling in a first portion of a main feed exchanger against said high pressure product streams to form a two-phase first feed stream; phase separating the condensed portion of said cooled first feed vapor stream in a first feed separator to form a second feed liquid stream and a second feed vapor stream; cooling said second feed vapor stream in a cold feed exchanger to condense a portion thereof; separating said condensed portion of said second feed vapor stream in a second feed separator to form a third feed liquid stream and a third feed vapor stream; cooling said third feed vapor stream; and introducing said second and third feed liquid streams and said third feed vapor stream to said single distillation column at increasingly colder sections, respectively. 
     
     
       6. A process according to claim 5 wherein said feed contains carbon dioxide in an amount greater than 100 parts per million volume. 
     
     
       7. A process according to claim 6 wherein said circulating fluid comprises methane. 
     
     
       8. A process according to claim 6 wherein said circulating heat pump fluid comprises a mixed cryogenic refrigerant.